Disc drive memory systems have been used in computers for many years as storage space for digital information. Information is recorded on concentric memory tracks of magnetic discs that rotate around a spindle. Information is accessed by read/write heads located on a pivoting arm which moves radially over the surface of the disc. The read/write heads (-transducers-) must be accurately aligned with the storage tracks on the disc to ensure proper reading and writing of information.
The discs are rotated at high speeds in an enclosed housing by means of an electric motor located inside the hub or below the discs. Such a motor is commonly known as a spindle motor. Such spindle motors typically have a spindle mounted by means of two ball bearing systems to a motor shaft in the hub. One of the bearings is located near the top of the spindle and the other near the bottom. These bearings allow for rotational movement between the shaft and the hub while maintaining accurate alignment of the spindle and the shaft. The bearings are normally lubricated by grease or oil.
The conventional bearing system described above is prone, however, to several shortcomings. First, vibration is generated by the balls rolling on the raceways. Ball bearings used in hard disk drive spindles run under conditions that often cause physical contact between raceways and balls in spite of the lubrication layer provided by the bearing oil or grease. Hence, ball bearings running on the apparently even and smooth, but microscopically uneven and rough, raceways transmit surface and circular imperfections in the form of vibration to the rotating disk. This vibration results in misalignment between the data tracks and the read/write transducer. These imperfections reduce the lifetime and effectiveness of the disc drive system.
Another problem is related to the use of hard disk drives in portable computer equipment and the resulting requirements for shock resistance. Shocks create relative acceleration between the disks and the drive casting. Since the contact surface in ball bearings is very small, the resulting contact pressures may exceed the yield strength of the bearing material and leave permanent deformation and damage on raceways and balls.
Moreover, mechanical bearings are not always scaleable to smaller dimensions. This is a significant drawback since the tendency in the disc drive industry has been to continually shrink the physical dimensions of the disc drive unit, as well as operate the units at ever-increasing speeds.
Another problem is that of potential leakage of grease or oil into the atmosphere of the disc drive, or outgassing of the components into this atmosphere. Because of the extremely high tolerance required for smaller radial spacing between tracks on the disc and the gap in the transducer which is used to read and write data on the disc, discs are located within sealed housings in which contaminants cannot be tolerated.
As an alternative to conventional ball bearing spindle systems, hydrodynamic bearing spindle systems have been developed. In these types of systems, lubricating fluid (gas or liquid) functions as the bearing surface between a stationary base housing and the rotating spindle/hub. For example, liquid lubricants including oil, more complex ferro-magnetic fluids, or even air have been utilized in hydrodynamic bearing systems. Air is popular because it is important to avoid the outgassing of contaminants into the sealed area of the head disc housing. However, air cannot provide the lubricating qualities of oil or the load capacity. Its low viscosity requires smaller bearing gaps and therefore higher tolerance standards to achieve similar dynamic performance. The liquid lubricant must be sealed within the bearing to avoid a loss which would result in reduced bearing load capacity and life. Otherwise, the physical surfaces of the spindle and of the housing would come into contact with one another leading to accelerated wear and eventual failure of the bearing system.
In the prior art, seals for containing the fluid within the disc drive utilize a pressurized film on the surface of the liquid-air interface, or surface tension. In the case of bearing assemblies which employ ferro-magnetic fluids, the seal is achieved by means of a magnetic field established at each end of the bearing.
A shortcoming of known hydrodynamic bearings includes the fact that many prior art hydrodynamic bearing assemblies require large or bulky structural elements for supporting the axial and radial loads, as many hydrodynamic bearings do not have the inherent stiffness of mechanical bearing assemblies. It is difficult to scale the structural support elements to fit within the smaller disc drive dimensions currently in demand. In other instances, hydrodynamic bearing assemblies require extremely tight clearances and precise alignments. This burden makes it difficult to manufacture such assemblies since even a small deviation or aberration can lead to faulty bearings. Further, as the gaps in which the fluid is located become smaller, the power consumed to rotate the spinning elements increases.
Another consideration is that the data track density on hard discs has been decreasing, and track mis-registration, commonly known as TMR, is becoming increasingly critical. One of the primary contributors to TMR is disc spindle runout. It consists of both repeatable runout and non-repeatable runout, commonly referred to as NRRO. The NRRO of a ball bearing motor is often too high for today's disc drives. However, fluid dynamic bearings (FDB) provide a much lower NRRO, which better supports the high aerial data densities of current disc drive technology.
The architecture of fluid dynamic spindles in the past has generally included a shaft in a housing, which provides radial stiffness, and a thrust bearing, which controls the vertical position of the spindle. Both the shaft and thrust bearing have generally been cushioned by a fluid film. The journal and shaft surfaces have typically been provided with miniature grooves, which create pressure by directing the fluid into specific areas of the journal.
One problem with this conventional FDB spindle motor arrangement is that it limits the degree to which the height of the motor assembly may be reduced. This is because the thickness of the thrust bearing must be added to the length of the shaft, which itself must be of a length sufficient to provide rotational stability. Moreover, use of a thrust bearing increases the amount of drag the motor must overcome during operation, increasing power consumption.
In the field of fluid dynamic bearing motors for use in hard disc drives, some prior systems including, but not limited to, small form factor motor designs for mobile applications have been limited by stringent power requirements. In the traditional “single-plate” FDB design, a thrust plate with two equal and opposing thrust bearings is affixed to the journal bearing shaft to provide axial stiffness. This approach results in bear gaps at large diameters, thereby increasing bearing drag and overall motor power.